Myron L 729II - Manual (Page 30)

4. CONVERTING A CURRENT
TO A VOLTAGE
Current measuring devices actually measure voltage, but have an
internal resistor as shown in figures III.B.1 & III.B.2 and are
scaled to display in current. If you have a voltage device and you
wish to utilize the current from the 4A (4-20mA) Module, the
following will help you to make this conversion.
To convert a 4-20 mA current output to operate or drive a voltage
input device it is necessary to install a LOAD resistor across the
input terminals as shown in figures III.B.6 & III.B.7. The value of
the resistor is chosen to match the input voltage range, i.e. 0-10
Volts requires a 500Ω resistor and will produce a 2 to 10 input
voltage. This floating zero is useful to indicate a broken 4-20 input
wire when the indication is zero volts.
For other input ranges, divide the input voltage range by 0.02, the
answer will be in ohms.
NOTE: The tolerance of the load resistor directly affects the
accuracy of the resulting voltage, i.e. 5% resistor = 5% error.
26
For an input voltage range of:
10.0 Volts the resistor value is 500 ohms indicates 2.0 V @ 4 mA.
5.0 Volts “ 250 ohms “ 1.0 V “
1.0 Volts “ 50 ohms “ 0.2 V “
100 millivolts “ 5 ohms “ 20 mV “
10 millivolts “ 0.5 ohms “ 2 mV “
Figure III.B.7
(+)
(-)
+
-
(+) SIGNAL OUT
POWER OUT
POWER IN +
Remote - Powered
VOLTAGE INPUT INSTRUMENT
+ 35 VDC
MAXIMUM
N
EXTERNAL LOAD
RESISTOR
Figure III.B.6
(+)
(-)
+
-
(+) SIGNAL OUT
POWER OUT
POWER IN +
Self - Powered
N
EXTERNAL LOAD
RESISTOR
SO
P
O
P
I
SO
P
O
P
I